alginate slowed because of network formation. Because of the dissolution
of alginate on the surface of the films, crosslinked films
were thinner than the control film. During the crosslinking process,
the greater the dissolution of internal alginate, the higher the
shrinkage rate and the film thickness. When films were crosslinked
with Al3+, a dense layer was quickly formed on the surface of the
films, with the consequence of lower internal alginate dissolution,
which led to a decreased shrinkage rate and film thickness. Zinc
ions because of their smaller ionic radius and atomic volume could
diffuse more quickly into the interior of films, at the same time,
due to lack of selectivity, there was more content in the interior of
the films. Both factors resulted in lower internal alginate dissolution
compared to Ca2+-crosslinked films. Therefore, the shrinkage
rate and thickness of Zn2+-crosslinked films were lower than Ca2+-
crosslinked films. Manganese ion, because of its low affinity toward
alginate, may only interact with one carboxyl site, therefore, upon
washing, the crosslinked films stretched, leading to a decease in
thickness and an increase in films’ area